Allow for multivalued server attributes
    
    In order to achieve the task, the following changes were required:
    
    * vectorize the base class for server attributes
    * add a child class that enforces single-value attributes. It still
      accepts/returns single-value lists in order to not break Liskov
      substitution principle
    * Existing attributes inherit from the child class
    
    https://pagure.io/freeipa/issue/6937
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>